This invention relates to the purification of Vinyl Chloride Monomer and the removal of water in Vinyl Chloride Monomer finishing.
In purifying Vinyl Chloride Monomer (VCM) produced by the cracking of 1,2 dichloroethane (EDC) according to well-known commercial manufacturing processes, trace amounts of water must be handled. This trace water either (a) is formed in the cracking process, (b) results from small amounts of water present in the EDC fed to the cracking furnaces used in the cracking process, or (c) is formed in-situ within the distillation process. Hydrochloric acid (HCl) is formed as a by-product or co-product in the production of VCM from EDC; and this HCl, when mixed with water, forms a mildly corrosive mixture. However, when the overall water concentration exceeds the solubility limit of water in VCM, the VCM becomes saturated and the water enters into a free phase state in HCl; this separate free-water phase is highly corrosive in comparison to the phase where the water concentration is below the saturation limit for VCM.
A drying operation can be used to remove water from an admixture of vinyl chloride, HCl, and water where the vinyl chloride is present either in substance or in trace quantity. One such drying system is described in U.S. Pat. No. 5,507,920 entitled xe2x80x9cProcess And Apparatus For Purifying Vinyl Chloridexe2x80x9d which issued to P. Schwarznaier, P. Kammerhofer, M. Stxc3x6ger, H. Kalliwoda, and I. Mielke on Apr. 16, 1996. This patent describes both the use of an evaporator and an optional molecular sieve or silica gel desiccant in drying water from a stream of HCl, water, and entrained vinyl chloride which has been distilled away (as an overhead vapor stream from a HCl/VCM distillation column, the third distillation column in a three column vinyl chloride separation system) from a feed stream rich in vinyl chloride and also containing HCl and entrained water. The patent describes that xe2x80x9cthe greatest water concentration prevails at the top ofxe2x80x9d the third HCl/VCM distillation column in that three column vinyl chloride separation system and that, accordingly, the xe2x80x9cdryingxe2x80x9d system is installed at the beginning of the vapor line recycling HCl and entrained vinyl chloride to the feed stream of the first distillation column of that three column vinyl chloride separation system.
The insertion of a drying system in the output vapor stream of a process line has some drawbacks, however. Any breakdown or plugging of such a drying system can rapidly affect the fluid dynamics in the HCl/VCM distillation column generating the vapor stream. Also, vapor streams tend to need physically larger equipment than liquid streams where the same mass of material is being handled; and, respective to the larger equipment, it requires more capital to install a vapor handling system than a liquid system respective to handling of the same mass of material. The use of a liquefaction system for the vapor stream can effectively solve some of the above issues, but this also requires capital and a cooling system to remove heats of vaporization and superheating. A true solution to the issue of water removal, therefore, is to provide a drying system which (1) removes water rapidly and efficiently from the VCM purification system at a location having a relatively high water concentration, (2) does not impact or potentially adversely affect the fluid dynamics in the HCl/VCM distillation column, (3) provides for a safe operating environment, and (4) minimizes the amount of capital needed to effect acceptable water removal from the vinyl chloride monomer purification system. The present invention provides a solution to these needs.
The invention in summary provides a method of removing water in a vinyl chloride monomer purification system by steps comprising
(1) providing a distillation column (having a top, a bottom, and a midsection) for separation of a liquid admixture of vinyl chloride, hydrogen chloride, and water into (a) an essentially pure vinyl chloride product stream and (b) a hydrogen chloride distillate stream; and
(2) placing a drying system in fluid communication with the distillation column midsection at a location where the water is at sufficient concentration to provide a useful mass transfer flux of water from a withdrawn midsection stream into a drying agent.